Field of the Invention
The present invention relates to an infrared transmission filter and an imaging device, such as an infrared camera employing the same.
Discussion of Background
Devices, which utilize light in a specific infrared wavelength range of from 0.7 μm to 2.5 μm (hereinbelow, also referred to as “infrared light”), for example, infrared remote controllers for home electric appliances, such as TV sets, audio products and air conditioners, infrared communication for exchange of information between cellular phones or personal computers, and infrared cameras for night photography or for security purposes, or motion sensors, have increased for recent years. This trend has increased the demand for filters, which cut off light in a visible wavelength range (hereinbelow, also referred to as “visible light”) serving as noise in these devices, and which selectively transmit only infrared light. Further, such infrared cameras are required to have a wide range of viewing angle to provide the cameras with an incident angle of about 0±40°. The incident angle is defined such that the incident angle is at 0° with respect to an optical axis.
Heretofore, as the infrared transmission filters, there have been known, e.g. one type made of a glass base material having a multilayer dielectric film vapor-deposited thereon so as to reflect visible light, another type made of glass having metal, such as cadmium (Cd), doped thereinto, and another type made of a transparent resin, such as an acrylic resin or a polycarbonate resin, containing pigment having a property of absorbing visible light.
However, such reflection filters, which are constituted by a glass base material having a multilayer dielectric film vapor-deposited thereon, have a problem of incident angle dependence in that the cut-off characteristics varies according to the incident angle of visible light. Such visible light absorbing glass filters having metal doped therein have recently tended to be avoided since heavy metal, such as Cd, is utilized. Further, visible light absorbing glass filters are not suited for an application where size reduction and weight reduction are needed, such as cameras, because of having a large weight and difficulty in thickness reduction. Furthermore, such visible light absorbing glass filters have a problem in that they are expensive.
On the other hand, such organic filters constituted by a transparent resin containing pigment are lightweight and can realize thickness reduction, and have no incident angle dependence unlike such reflection filters. However, such type of existing filters are insufficient in permselectivity for infrared light closer to visible light. Specifically, such type of existing filters have a poor cut-off characteristics for light in a wavelength range of from 730 to 830 nm. For this reason, this type of existing filters have not been put into practice because of having a small S/N ratio in e.g. an application where an object is irradiated with light having a wavelength of from 830 to 850 nm, and the light reflected from the object is captured by an infrared camera.